I think you are referring to payroll withholding rather than income tax.

I'm sure you know much more about this than I do, but I'm talking about income taxes on bonuses.

As I understand it, bonuses are usually taxed in two ways.

Either they are taxed as "supplemental income" which gets hit with a flat 28%(?) federal withholding (plus state and payroll taxes)--that 28% is higher than most people's average tax rate, so it feels like it is getting taxed harder.
Or, they are combined with a paycheck. If you normally earn $3000 a month, get a 10k bonus and they are lumped together into one $13,000 check, my understanding is that check will be taxed as if that was your new monthly wage. So you will be hit with the taxes paid by someone earning $156k rather than someone earning 36k.

Obviously, when you actually file your taxes, it ends up working out fine. Your bonus gets treated as regular income and you pay more/get a larger rebate depending on how much you have paid so far.